Telegraph circuit



H. NYQUIST May 28, 1929.

TELEGRAPH CIRCUIT Filed Dec. 9, 1927 IN V EN TOR.

\@,X I I I E 8 BY W 1 ATTORNEY Patented May Y NYQUIST, or iarnnnuan, NEWanasnr, Assienon 'ro AMERICAN 'rnnnrnonn I aria? ant oas s.

\ AND TELEGRAPH COMPANY, A CORPOTION E NEW Y0.

TELEGRAPH CIRCUIT.

Application filed December 9, 1927. Serial No. 238,803.

This invention relates to telegraph systems, and more particularly toimprovements in a method of and means for insuring symmetrical signalsand, hence, lack of distortion in the loop circuits of such systems.

It has been found that it is a common experience with telegraphsubscribers loops that the wave shape is not the same on opening as itis on closing the subscribers key.

This lack of symmetry gives rise to distortion of the signalstransmitted by the subscriber and in some instances it has beenfoundnecessar to go to considerable lengths to obviate t ese effects. It isthe primary object of this invention to present and illustrate a methodfor insuring symmetrical signals in tele raph subscribers loop circuits.The met 0d of the arrangements of the invention comprises soconstructing the loop circuit that its impedance as seen from thesubscrib ers key is a constant resistance at all fre quencies. It willbe seen from the discussion hereinafter given that this will result insymmetrical signals in the loop circuit. Other objects and features ofthe invention will appear more fully from the detailed description ofthe invention hereinafter iven. The invention may be more fullyunderstood from the following description, together with the accompanyindrawing, in

the Figures 1 to 8, inclusive, ot which the invention is illustrated.Figs. 1, 2oand 8 are schematic showings of telegraph loop circuitsembodying the principles of the invention. Figs. 3, 4, 5, 6 and 7 arecurves showing the wave shape of the signals. Like reference charactershave been utilized to denote like parts in all of the figures.

In Fig. 1 is shown symbolically a telegra h loop circuit comprising-thebattery B, key and inductance L to represent the inductance of a seriesrelay, or sounder. To illustrate the method of the invention, whichconsists in so constructing the loop that its impedance as seen from thesubscribers key is a constant resistance at all frequencies, it has beenassumed that all the reactances in the loop'are negligible exceptinductances of series relays or sounders. Furthermore, only one suchseries relay, as illustrated by the inductance L is shown. Theindividual relays or sounders will now be equipped with compensatingshunts. For example, let a given relay have the inductance L and theresistance 1'. It may be convenient as a preliminary to add aresist'ance as shown in series to bring the total reslstance up to B.When the shunt which is connected in parallel with the combination ofrelay and resistance consists of a condenser and resistance in series,the resistance is given the value It and the condenser the value C. Theimpedance of branch LR as seen from key K is ZLR and the impedance ofbranch GR is,

- a E" to The impedance of the combination as seen from K would besymmetrical in shape, reference is had to Fig. 2. In the loop of Fig. 2,the opening of key K efiects the instantaneous insertion of aresistanceless battery B into the circuit whose voltage is so chosen asto bring the current to zero. This is true because since the circuitlooks like a resistance at this point, there are no transients in thebattery branch, or in other Words, that portion of the circuit externalto points A and B. Furthermore, the closing of the key K removes thisbattery. Accordingly, in the loop circuit of Fig. 2, considered externalto the points A and B, the opening and closing of the key is equivalentto varying the potential in a constant impedance circult in equal andopposite steps and will give .rise to a symmetrical wave of the shapeshown in Fig. 3. It is obvious that in the loop of Fig. 1, consideredexternal to points A and B, the openingand closing of key K isequivalent to the operation already described with respect to Fig. 2 inthat it will also give rise to a symmetrical wave of the shape of thecurve of Fig. 3. Accordingly, the two loop circuits external to thepoints A and B are equivalent.

Considering that branch of the loop in Fig. 2 between the points A and Bdesignated b LR, it will be seen that the closing of key l results inapplying a positive potential across terminals A and B. This would giverise to a wave in branch LR of the form shown by the curve of Fig. 4.The application of a negative, instead of a positive, potential acrossterminals A and B would obviously give rise to a wave of the same formbut in reverse direction, .as shown by the curve of Fig. 5. Let usassume a direct current I flowing through the branch LB- due to apositive potential across terminals A and B in Fig. 2. If we now,without removing this positive potential, apply an equal negativepotential by closing key K in Fig. 2 at the time indicated by point P,the wave shape will be illustrated by the curve of Fig. 6. This curve isobviously of the same form as that of Fig. 5 except that it starts fromI instead of zero. The continued opening and closing of key K would giverise to a signal wave of the form illustrated in Fig. 7, in which thebuilding up of the wave is symmetrical to its decay. In other words, theopening and closing of the key in Fig. 2, which is equivalent to varyingthe potential in equal and opposite steps in the loop circuit which hasbeen made of constant impedance will give rise to a symmetrlcal wave.

It has been pointed out heretofore that the two loop circuits of Figs. 1and 2 considered external to points A and B are equivalent. But the twoloop circuits between points .A and B are identical with each other.Accordingly, as they are equivalent outside of points and B and areidentical between points A and B, the operation between points A and Bmust be the same. Accordingly, the operation of key K of Fig. 1 willgive rise to a wave of the form shown in Fig. 7, in

which the building up of the wave is symmetrical to its decay. In otherwords, the wave shape of the signals in the relays will be symmetricaland distortion will be obviated.

It is pointed out that the above discussion has dealt with only onerelay or sounder such as illustrated by the inductance L..

However, if a plurality of relays or sounders existed, each would beprovided with a compensating shunt comprising a condenser and aresistance so that the impedance would equal a constant resistance atall frequencies.

It might be desirable to make certain adustments in thatportion of theloop looking in the other direction from the key K, such as to the left,so that the impedance looking in this direction might be equal to aconstant resistance at all frequencies. Accordingly, reference may behad to Fig. 8. In this loop circuit, an inductance L is shown. Thismight represent the sending relay. The equivalent capacity betweenconductors may be termed C. The d. c. conductor resistance the spirit ofthe invention as defined by the appended claims. 1

What is claimed is:

1. The method of insuring symmetrical signal waves in telegraphcircuits, which comprises constructing the circuit so that its impedanceas seen from the telegraph key or sending contacts is a constantresistance at all frequencies.

2. A telegraph circuit comprising a key, a resistance and a relay havinga certain inductance in series in said circuit, and a shunt circuitaround said relay and resistance, said shunt including a condenser and aresistance, the values of said inductance and capacity and resistancesbeing so chosen that the impedance of the combination as seen from saidkey will equal a constant resistance at all frequencies.

3. A telegraph circuit comprising a ke a resistance B and a relay havingan in uctance L in series in said circuit, and a shunt circuit aroundsaid relay and resistance, said shunt including a condenser C and aresistance R, the values of C, L and B being so chosen that L/U=R 4:.The method of insuring symmetrical signal waves in telegraph circuits,which comprises constructing the circuit so that its impedance as seenfrom a telegraph key or sending contacts in either direction is aconstant resistance at all frequencies.

In testimony whereof, I have signed my name to this specification this7th day of December, 1927.

HARRY NYQUIST.

